Metal vapour laser

ABSTRACT

There is disclosed a metal vapour laser comprising a discharge tube having a buffer gas therein and operating at high temperature, the buffer gas including a laser output power enhancing substance in an amount sufficient to substantially increase the power output of the laser. There is also disclosed a process for operating a metal vapour laser of the invention.

TECHNICAL FIELD

This invention relates to metal vapour lasers and to methods foroperating metal vapour lasers.

BACKGROUND ART

Pulsed metal vapour lasers are a class of cyclic pulsed laser whichgenerate high average power at high pulse repetition rates (kilohertz totens of kilohertz) in the visible and infrared regions of the spectrum.They have been known since 1966 and are utilised commercially in a rangeof applications, particularly where relatively high power devices arerequired. Metal vapour lasers producing greater than 120 W are currentlyavailable. Such lasers find application in fields such as medicine,forensic science, machining, as pump sources for tunable dyes, and inisotope separation, for example in uranium enrichment.

The active region of a pulsed metal vapour laser is the discharge plasmatube, which is an extended tubular zone in which the metal vapour isconfined and through which a pulsed high-current electrical gasdischarge passes. The discharge plasma tube is normally formed fromrefractory ceramic material (usually recrystallized alumina) andsurrounded by high-temperature insulation. The discharge plasma tubeitself must be maintained at very high temperatures (for example1400-1700° C. for a copper vapour laser) to ensure adequate vapourpressure (by way of thermal evaporation) of the metal, which is usuallydistributed along the tube. A buffer gas, usually He or Ne, isinvariably present at a pressure of tens or hundreds of millibar tostabilise the metal vapour discharge.

Thus, in operation, metal vapour lasers typically include small piecesof the metal distributed in the plasma discharge tube, and, with thebuffer gas flowing slowly through the tube, it is heated externallyand/or by the discharge to a temperature such that the vapour pressureof metal in the buffer gas is sufficient to enable lasing to take place.For example, for a copper vapour laser the copper vapour density istypically about 1-10 Pa which requires a temperature of typically1400-1700° C.

Although previously known metal vapour lasers are typically capable ofoperating at relatively high efficiencies (up to about 1%) and producingrelatively high power output, there is a need for an improved metalvapour laser which provides higher output power than presently knownmetal vapour lasers, with at least comparable efficiencies, but which isrelatively simple to use and is capable of stable operation. Desirably,such an improved metal vapour laser would be capable of operating withno flowing buffer gas.

OBJECTS OF THE INVENTION

It is an object of this invention to provide an improved metal vapourlaser. It is a further object of this invention to provide an improvedprocess for operating a metal vapour laser. In particular, it is anobject of the present invention to provide a process for operating ametal vapour laser, by including in the laser one or more additives, toimprove the output power and output beam characteristics of the laser incomparison to known high temperature metal vapour lasers.

SUMMARY OF THE INVENTION

According to a first form of the present invention, there is provided ametal vapour laser comprising a discharge tube having a buffer gastherein and operating at high temperature, the buffer gas including alaser output power enhancing substance in an amount sufficient tosubstantially increase the power output of the laser.

According to a second form of the present invention, there is provided aprocess for operating a metal vapour laser comprising a discharge tubehaving a buffer gas therein and operating at high temperature, utilisinga buffer gas which includes a laser output power enhancing substance inan amount sufficient to substantially increase the power output of thelaser. As described below, the buffer gas may have the laser outputpower enhancing substance premixed therewith, or the laser output powerenhancing substance may be generated in the discharge tube under theopening conditions of the laser.

Thus, the second form of the invention provides a process for operatinga metal vapour laser comprising a discharge tube having a buffer gastherein and operating at high temperature, comprising premixing a laseroutput power enhancing substance with the buffer gas or generating alaser output power enhancing substance in the discharge tube, the laseroutput power enhancing substance being present in the discharge tube atan operating condition of the laser in an amount sufficient tosubstantially increase the power output of the laser.

It is presently theorised by the inventors that the laser output powerenhancing substance acts as an electron scavenger in the active regionof the laser when the laser is in operation, though the inventors do notwish to be bound by this theory.

As used herein, the expression “an amount sufficient to substantiallyincrease the power output” in connection with a laser or the operationof a laser, means an amount which, when included in the buffer gas ofthe operating laser, results in a substantial increase in the poweroutput of the laser compared to the power output of the laser when it isoperated under the same conditions in the absence of the laser outputpower enhancing substance.

Typically the metal vapour of the metal vapour laser of the presentinvention is a copper vapour, gold vapour, manganese vapour, cadmiumvapour, zinc vapour, mercury vapour, tin vapour, magnesium vapour,barium vapour, chromium vapour, iron vapour, cobalt vapour, nickelvapour, silver vapour, gallium vapour, indium vapour, europium vapour,thallium vapour, bismuth vapour, antimony vapour, tellurium vapour,selenium vapour, strontium vapour, calcium vapour or a lead vapour.

More typically, the metal vapour is selected from copper vapour, goldvapour, manganese vapour, europium vapour, thallium vapour, bariumvapour, iron vapour, bismuth vapour, strontium vapour, calcium vapourand lead vapour. Even more typically, the metal vapour is a coppervapour.

Generally, the operating temperature of the metal vapour laser issufficient to provide a partial pressure of metal vapour in the lasertube of from about 13 Pa to about 130 Pa. For a copper vapour laser, forexample, the operating temperature is from about 1400-1700° C., usuallyfrom 1400-1600° C., while for a lead vapour laser it is from about900-1100° C. and for a gold vapour laser it is from about 1550-1850° C.Operating temperatures for other metal vapour lasers are known topersons skilled in the relevant art.

Typically, the laser output power enhancing substance is a speciescomprising one or more atoms selected from oxygen, sulfur, fluorine,chlorine, bromine and iodine. For example, the laser output powerenhancing substance may be fluorine; chlorine; bromine; iodine; ahydrogen halide such as HF, HCl, HBr or HI; H₂O; H₂S; SF₆; BF₃; oxygen;sulfur; a halogenated hydrocarbon such as methyl chloride, methylbromide, dichloromethane, trichloromethane, tetrachloromethane,trichloroethane, trichloroethene, tetrachloroethane, tetrachloroetheneor any of the “freons”; a mixture of two or more of the foregoing; or aspecies derived from any of the foregoing under the operating conditionsof the laser. Typically the laser output power enhancing substance isprovided in a mixture of one or more of the foregoing with hydrogenand/or an additive such as a hydrogen source, or a species derivedtherefrom.

The buffer gas is typically an inert gas, such as krypton, xenon, argon,helium or neon or a mixture of two or more thereof, or a mixture of aninert gas with hydrogen or deuterium. More typically, the buffer gas isselected from neon and helium.

The pressure of the buffer gas depends on which gas is selected as theinert gas. Usually, the pressure of the buffer gas in the operatinglaser ranges from 0.1 kPa to 20 kPa, more usually from 0.5 kPa to 15kPa, or from 1.3 kPa to 13 kPa, or from 2 kPa to 10 kPa or from 3 kPa to7 kPa, or from 5 kPa to 6 kPa. Even more usually the pressure of thebuffer gas is about 5.2 kPa when the buffer gas is predominantly neon.

In a third form of the present invention there is provided a process foroperating a metal vapour laser comprising a discharge tube having abuffer gas therein, the buffer gas including a laser output powerenhancing substance in an amount sufficient to substantially increasethe power output of the laser; comprising the step of adjusting theconcentration of the laser output power enhancing substance by adding tothe buffer gas an additive capable of controlling the concentration ofthe laser output power enhancing substance in the buffer gas.

In a fourth form of the present invention, there is provided a metalvapour laser comprising a discharge tube having a buffer gas therein,the buffer gas including a laser output power enhancing substance in anamount sufficient to substantially increase the power output of thelaser, and means operatively associated with said discharge tube to addto the buffer gas an additive capable of controlling the concentrationof the laser output power enhancing substance in the buffer gas.

Typically, the additive is hydrogen or an isotope thereof, such as H₂,D₂, T₂, HD, HT or DT, water (usually as a vapour), or a hydrocarbon,such as methane, ethane, ethene, ethyne, propane, propene, propyne, anyof the isomeric butanes, butenes, butynes, pentanes, pentenes, pentynesor higher aliphatic hydrocarbons, or aromatic hydrocarbons such asbenzene, tolune, a xylene or a higher homologue, or a mixture of any twoor more of the foregoing, or deuterated forms of the foregoing. Moretypically, the additive is hydrogen or deuterium, still more typicallyhydrogen. Yet more typically, the laser output power enhancing substanceis HBr, HCl, or a mixture of HCl and HBr, and the additive is hydrogen.

One way is which the concentration of laser output enhancing substancemay be varied in a process according to the third form of the inventionis to vary the concentration of additive added to the buffer gas.Alternatively, the concentration of the additive may be fixed and theconcentration of the laser output power enhancing substance varied byvarying the concentration of a precursor of the laser output powerenhancing substance, as described in more detail herein below.

The amount of laser output power enhancing substance present in thebuffer gas depends on the metal vapour of the laser but, given theteaching herein, may be determined by a person of ordinary skill in theart with no more than ordinary experimentation. When the metal vapour isa copper vapour, for instance, or the vapour of many of the other metalsexemplified herein, the laser output power enhancing substance isgenerally present in an amount of from a trace to about 5% by volume ofthe buffer gas, usually from 0.1% to 5% by volume, more usually from0.2% to 4% by volume, yet more usually from 0.25% to 3% by volume, stillmore usually from 0.3% to 2.5% by volume, and even more usually from 0.5to 2% by volume. For example, the laser output power enhancing substanceis typically present in the range of from 0.5-1, or 1-1.5, or 1.5-2, or2-2.5, or 0.5-1.5, or 1-2, or 1.5-2.5 or 2-3, or 2.5-3.5% by volume.

The additive is generally present in an amount of from 0.1 to 5% byvolume, more usually from 0.2% to 4% by volume, still more usually from0.25% to 3% by volume, even more usually from 0.3% to 2.5 % by volume,yet more usually from 0.5 to 2% by volume. For example, the additive istypically present in the range of from 0.5-1, or 2.5% by volume, andeven more usually from 0.5 to 2% by volume. For example, the laseroutput power enhancing substance is typically present in the range offrom 0.5-1, or 1-1.5, or 1.5-2, or 2-2.5, or 0.5-1.5, or 1-2, or 1.5-2.5or 2-3, or 2.5-3.5% by volume.

The additive is generally present in an amount of from 0.1 to 5% byvolume, more usually from 0.2% to 4% by volume, still more usually from0.25% to 3% by volume, even more usually from 0.3% to 2.5% by volume,yet more usually from 0.5 to 2% by volume. For example, the additive istypically present in the range of from 0.5-1, or 1-1.5, or 1.5-2, or2-2.5, or 0.5-1.5, or 1-2, or 1.5-2.5 or 2-3, or 2.5-3.5% by volume.Even more typically the laser output power enhancing substance is HCl inan amount of from about 0.2% to 1% by volume of the buffer gas and theadditive is hydrogen in an amount of from about 1% to 2% by volume ofthe buffer gas. Usually in these forms of the invention the partialpressure of the additive and/or the laser output power enhancingsubstance in the buffer gas is from about 1 Pa to about 2000 Pa, moreusually from 2 Pa to 1500 Pa or 3 Pa to 1200 Pa or 4 Pa to 1000 Pa or 5Pa to 950 Pa or 6 Pa to 900 Pa or 7 Pa to 850 Pa or 8 Pa to 800 Pa or 10Pa to 750 Pa or 12 Pa to 700 Pa, even more usually about 13 Pa to about665 Pa.

Without wishing to be bound by theory, the inventors speculate that whenthe laser output power enhancing substance is a halogen containingsubstance the additive acts by chemically reducing halogen containingspecies present in the discharge tube into hydrogenated species such asHCl and HBr which, in appropriate concentrations, are more effectivethat the unreduced halogen containing substance for increasing the poweroutput of the laser.

The additive may be premixed with the buffer gas and admitted to thedischarge tube, or it may be generated in situ in the discharge tube.Where the additive is a gas, it may be supplied from a pressurisedsource such as a gas cylinder and mixed in an appropriate amount withthe buffer gas. Alternatively, the additive may be stored, for example,in an adsorbed or adsorbed form on a convenient adsorbent or adsorbentsuch as activated carbon, alumina, silica, zeolite or metal, such aspalladium, or in the form of a chemical compound which is capable ofdecomposing or dissociating at an elevated temperature to regenerate theadditive. In such situations, the additive is typically obtained byheating the adsorbent or chemical compound in an atmosphere or flowingstream of the buffer gas. The concentration of additive in the buffergas may be adjusted by adjusting the heating temperature and/or the flowrate of the buffer gas. Similarly, if the additive is a liquid atambient temperatures, a mixture of the vapour of the additive and thebuffer gas may be obtained by flowing the buffer gas through the liquidor over its surface and the concentration of the additive in the buffergas may be adjusted by controlling the temperature of the liquid and/orthe flow rate of the buffer gas.

In yet a further alternative, the concentration of additive, when it ishydrogen or an isotope thereof, may be controlled by a “getter” in theplasma region as described in more detail hereinbelow.

Conveniently, when the additive is hydrogen or an isotope thereof, itmay be added to the buffer gas directly as a gas or it may be stored ina “getter” as a metal hydride, deuteride, etc. and generated in situ orexternally to the discharge tube. Suitable metals for forming metalhydrides which dissociate to regenerate hydrogen are known and includepalladium, lanthanum, yttrium, erbium, cerium and other rare earthmetals, uranium, scandium, vanadium, titanium, zirconium, tantalum,niobium, chromium, manganese, iron, cobalt, nickel, thorium, copper,magnesium and alloys of two or more thereof, such as “Mischmetal”,“Mischmetal”-Ni, LaNi₅, Mg₂Ni, FeTi, Fe—Ti—Mn, Fe—Ti—Cr, Fe—Ti—Co,AlTh₂, CaAg₂, Ti—Mn, Ti—Cu, Ti—Ni, Zr—Ni, V—Nb, Mg₂Cu and Zr—U.

When the additive is generated in situ in the discharge tube, this maybe achieved by including in the discharge tube a quantity of theadditive adsorbed or absorbed on an adsorbent or absorbent as describedabove or in the form of a chemical compound which is capable ofdecomposing or dissociating at an elevated temperature to regenerate theadditive.

The laser output power enhancing substance may be introduced into thebuffer gas in a variety of ways. For instance, it may be mixed with thebuffer gas externally to the laser discharge tube, and the mixture thenintroduced into the tube.

As a further possibility, halogen or hydrogen halide may be pre-adsorbedor adsorbed onto a zeolite or other solid adsorbent or adsorbent and thepretreated adsorbent or adsorbent included in the discharge tube. Onheating the discharge tube, the adsorbed halogen and hydrogen halide isdesorbed into the buffer gas in the discharge tube. The concentration oflaser output power enhancing substance in the plasma region of the laseris then typically controlled by the inclusion of a suitable additivesuch as hydrogen in the buffer gas.

As yet a further possibility, the laser output power enhancing substanceor its precursor may be a substance which is a solid which vaporizesand/or dissociates under the operating conditions of the laser, andwhich is included in the discharge tube as a solid when the dischargetube is cold. Examples of such substances include ammonium halides andhydrohalide salts of organic amines.

In another embodiment of the invention, the laser output power enhancingsubstance is generated in situ. In one form of this embodiment, asubstance is included in the buffer gas in contact with the interior ofthe discharge tube for a pre-conditioning period prior to initiating adischarge in the buffer gas, the substance being capable of reactingwith or being adsorbed or adsorbed on the surface of the laser dischargetube. The substance may be any of the substances exemplifiedhereinbefore as laser output power enhancing substances or substancesfrom which a laser output power enhancing substance is derived under theoperating conditions of the laser (herein termed a “precursor of a laseroutput power enhancing agent”), or it may be a substance which reactswith the material from which the discharge tube is constructed so as toproduce a laser output power enhancing substance of a precursor of alaser output power enhancing substance. In this embodiment, after thepre-conditioning period buffer gas admitted to the discharge tubeusually includes no laser output power enhancing substance or precursorof a laser output power enhancing substance. For example, when thedischarge tube is alumina and the precursor of the laser output powerenhancing substance includes hydrogen halide or halogen, it is believedthat aluminum halide is formed in the discharge tube during thepre-conditioning, and dissociates during the high temperature operationof the laser. By varying the reservoir of an additive such as hydrogenor an isotope thereof in the discharge tube, the rate at which hydrogenhalide is produced can be affected. Thus, typically no further hydrogenhalide or halogen is required to be included in buffer gas admitted tothe discharge tube after the pre-conditioning period.

In a variation of this embodiment of the invention, the laser dischargetube includes a metal halide in a quantity sufficient to substantiallyincrease the output power of the laser at the operating temperature ofthe laser. That is, the laser output power enhancing substance isderived from the metal halide under the operating conditions of thelaser. It will be appreciated that in this variation, it is notnecessary to pre-condition the laser before use.

For example, the laser may be provided with an amount of one or moremetal halides in the discharge tube. Typically, in this form of theinvention a concentration of hydrogen, or a mixture of hydrogen with ahydrogen halide, is included in the flowing buffer gas as describedabove. Usually a metal halide which is utilised in such an embodiment ofthe invention is a fluoride, chloride, bromide or iodide of a transitionmetal, a lanthanide, an actinide, an alkali metal, aluminum, zinc,cadmium, mercury, calcium, strontium or barium; such as AuCl₃, FeCl₃,HgBr₂, HgCl₂, HgF₂, NbBr₅, NbF₅,NbCl₅, OsF₅, TiCl₄, TiCl₃, TiBr₄, ZrCl₄,ZrBr₄, MoF₅, MoCl₅, NiCl₂, CoCl₂, WBr₅, WCl₅, WCl₆, AlCl₃, ReCl₅, ReCl₆,ReBr₄, PbBr₂, PbCl₂, TaCl₅, TaF₅, TaBr₅, TaI₅, SnBr₄, SnCl₂, SnCl₄,SnF₂, SnF₄, VCl₄, VCl₃, VCl₂, ZnBr₂, ZnBr₄, etc. More usually, the metalhalide is a metal chloride, even more usually TaCl₅.

The laser output power enhancing substance may be generated from themetal halide in any of a number of ways. For example, if the metalhalide was a sufficient vapour pressure at the operating temperature ofthe laser, sufficient laser output power enhancing substance or itsprecursor may be provided by vaporisation of the metal halide, and thequantity of laser output power enhancing substance may be controlled byadjusting the temperature of the discharge tube. Alternatively, a beamof high energy electrons may be directed at a quantity of the metalhalide located in the laser assembly, for example if the electron beamhas sufficient energy to dissociate the metal halide. As a furtherpossibility, the metal halide may be placed between a pair of electrodesin the laser assembly and subjected to a radio-frequency or dc dischargeby applying a radio-frequency ac potential difference or a dc potentialdifference across the electrodes.

Alternatively, a quantity of one or more pure metals or a metal oxide,hydroxide, carbonate or other salt capable of reacting with a gaseoushalogen-containing reagent to form a halide of the metal may be includedin the discharge tube and the tube pre-conditioned with a gaseoushalogen-containing reagent which is capable of reacting with themetal(s) or salt(s), optionally at an elevated temperature, for a periodof time sufficient to form an amount of a halide of the metals(s).

Thus, according to a fifth form of the present invention there isprovided a metal vapour laser comprising a discharge tube and capable ofoperating at high temperature, the laser comprising a quantity of afirst metal capable of providing a sufficient metal vapour pressure atthe high temperature to permit laser light to be produced by the laserat the high temperature, characterised in that the laser furthercomprises a quantity of a second metal or salt thereof, the second metalbeing different from the first metal, the second metal or salt beingcapable of reacting with a gaseous halogen-containing reagent to producea halide of the second metal, wherein the halide of the second metal, ora species derived therefrom under an operating condition of the laser,enhances the output power of the laser.

According to a sixth form of the invention there is provided a processfor operating a metal vapour laser comprising a discharge tube having abuffer gas therein and operating at high temperature, the processcomprising:

providing in the laser a quantity of a first metal and a quantity of asecond metal or salt thereof, the second metal being different from thefirst metal, the first metal being capable of providing a sufficientmetal vapour pressure at the high temperature to permit laser light tobe produced by the laser at the high temperature, and the second metalor salt thereof being capable of reacting with a gaseoushalogen-containing reagent to produce a halide of the second metal;

pre-conditioning the laser by contacting a gaseous halogen-containingreagent with the second metal or salt thereof in the discharge tube at atemperature lower than the high temperature for a time and underconditions sufficient for a halide of the second metal to be formed;

raising the temperature of the discharge tube to the high temperature;

passing a buffer gas through the discharge tube; and

generating a discharge in the discharge tube and producing laser lightfrom the laser.

Typically, the gaseous halogen-containing reagent is a halogen or ahydrogen halide, more typically a hydrogen halide such as HCl or HBr,still more typically hydrogen chloride. The second metal may be atransition metal, lanthanide, actinide, alkali metal, aluminum, zinc,cadmium, mercury, calcium, strontium or barium. Usually, the secondmetal is tantalum, zirconium, palladium, nickel, niobium, platinum,copper, aluminum, titanium, molybdenum, tungsten, lead, rhenium or tin.More usually, the second metal is tantalum.

The second metal may be provided as one or both of the electrodes of thelaser. That is, one or both of the electrodes of the laser may beconstructed of one or more of the transition metals, lanthanides,actinides, aluminum, zinc, cadmium, mercury, calcium, strontium orbarium: for example tantalum, zirconium, palladium, nickel, niobium,platinum, copper, aluminum, titanium, molybdenum, tungsten, lead,rhenium or tin. A quantity of the second metal may also be providedelsewhere in the laser, in which case one or both of the electrodes neednot include an amount of the second metal, and the electrodes may thenbe constructed of the first metal, or stainless steel, inconel, or othermetal generally known in the art for forming the electrodes of metalvapour lasers.

Thus, the second metal or salt thereof may be positioned in a region ofthe laser discharge tube assembly in which the second metal or saltthereof is contacted with the gaseous halogen containing reagent. In oneform of this embodiment of the invention the second metal is positionedin a region of the laser which reaches a temperature sufficiently highto provide an adequate vapour pressure of the metal halide and/ordissociated metal and halogen atoms in the discharge tube. In thisarrangement, the quantity of laser output power enhancing substance inthe laser may be controlled by adjusting the temperature of the laser.Alternatively, the second metal or salt thereof may be positioned in aregion of the laser where, after having been reacted with thehalogen-containing reagent, it may be bombarded with high energyelectrons or subjected to an ac or dc discharge as described hereinabove. Typically, the second metal is positioned in a space between thecathode of the laser and the discharge tube, or within an input line ofthe gaseous halogen-containing reagent, or impregnated into an insulatorwhich typically separates the discharge tube from an outer vacuum tube,or within an end-bell of the laser.

The second metal, or its salt, or a metal halide included in the lasermay be provided in the form of solid pieces of the metal, salt orhalide, or in powdered form, or impregnated into a suitable pourercarrier such as glass matting or fibrous ceramic material (including forexample an insulator surrounding the discharge tube of the laser) or asintered metal.

In one embodiment of the invention a laser discharge tube containing anamount of the second metal, such as tantalum, or a salt thereof, ispre-conditioned by flowing the gaseous halogen-containing reagent,typically a gas containing a hydrogen halide, usually hydrogen chloride,through the discharge tube at ambient temperature or at an elevatedtemperature. Following the pre-conditioning period, the laser may beoperated with a flow of inert gas passing through the discharge tube, ora flow of a mixture of an inert gas and hydrogen or other additive asexemplified herein above, or a flow of a mixture of an inert gas,additive such as hydrogen, and laser output power enhancing substance,typically hydrogen chloride. Usually, in this form of the invention, thelaser is operated after the pre-conditioning period with a mixture ofhydrogen and neon flowing through the discharge tube. More usually,immediately after pre-conditioning, the concentration of hydrogen in theflowing gas is very low or zero, and the concentration of hydrogen inthe flowing gas is then typically increased to about 2-3% by volume overthe stable operating life of the laser.

It has been found that the incorporation of a metal such as tantalum inthe laser as described above makes for easier and more precise controlof the operation of the laser, and its output is stable for extendedperiods.

It has further been found that the inclusion in the laser of a quantityof a third metal, different from the first and second metals, or of ametal halide as well as the second metal, further enhances the poweroutput of the laser. Thus, the invention also provides a laser inaccordance with the fifth form of the invention and further comprising aquantity of a substance selected from the group consisting of a thirdmetal and a metal halide, the third metal being different from the firstand second metals. Also provided is a process in accordance with thesixth form of the invention and further comprising providing in thelaser a quantity of a substance selected from the group consisting of athird metal and a metal halide, the third metal being different from thefirst and second metals. The metal halide may be nay of the metalhalides exemplified herein above. In this form of the invention, thesecond metal is typically provided as one or both of the electrodes ofthe laser, and is more typically tantalum. The third metal may be any ofthose metals disclosed herein above as suitable for the second metal,but is typically selected from the group consisting of Au, Fe, Hg, Nb,Os, Ti, Zr, Mo, Ni, Co, W, Al, Re, Pb, Ta, Sn, V and Zn.

In yet a further alternative embodiment of the invention, an intimatemixture of a metal and a solid halogen-containing reagent, typically ahalide of the metal, is included in the laser discharge tube. The solidhalogen-containing reagent is selected to be capable of reacting withthe metal in the discharge tube to produce a relatively volatile halideof the metal. Thus, for example, the laser discharge tube may beprovided with a quantity of tantalum metal and also a quantity oftantalum pentachloride. Similarly, in the case of the copper vapourlaser for example, a mixture of copper and cupric halide may be includedin the discharge tube. In this example, when the tube is heated,chemical reaction takes place between the copper and the cupric halide,forming cuprous halide which, on further heating, dissociates and formshalogen atoms. In this alternative also, hydrogen is typically includedin the buffer gas.

In still a further alternative embodiment of this form of the invention,a metal hydride and a metal halide are included in the laser dischargetube, the metal halide being capable of providing the laser output powerenhancing substance at the operating temperature of the laser, and themetal hydride being capable of providing an additive capable ofcontrolling the concentration of laser output power enhancing substanceon the discharge tube. Typically, the metal halide is one of the metalhalides exemplified herein above, and the metal hydride is typically ahydride of palladium, lanthanum, yttrium, erbium, cerium and other rareearth metals, uranium, scandium, vanadium, titanium, zirconium,tantalum, niobium, chromium, manganese, iron, cobalt, nickel, thorium,copper, magnesium and alloys of two or more thereof, such as“Mischmetal”, “Mischmetal”-Ni, LaNi₅, Mg₂Ni, FeTi, Fe—Ti—Mn, Fe—Ti—Cr,Fe—Ti—Co, Alth₂, CaAg₂, Ti—Mn, Ti—Cu, Ti—Ni, Zr—Ni, V—Nb, Mg₂Cu or Zr—U.

In these embodiments, where the discharge tube is pre-conditioned, thepreconditioning may be at ambient temperature or at an elevatedtemperature. Usually, the discharge tube is pre-conditioned at elevatedtemperature, more usually in the range 100-1000° C., even more usually200-950° C. or 300-930° C. or 400-900° C. or 500-890° C. or 600-880° C.or 700-870° C. or 800-860° C., still more usually about 850° C.Typically, the pre-conditioning is carried out with a mixture of inertgas and the precursor of the laser output power enhancing substance orgaseous halogen-containing reagent, or with the precursor of the laseroutput power enhancing substance or gaseous halogen-containing reagentalone. More typically, the pre-conditioning is carried out with theprecursor of the laser output power enhancing substance or gaseoushalogen-containing reagent alone. The pre-conditioning period istypically from 1 hour to 10 weeks, more typically from 2 hours to 1week, even more typically from 3 hours to 24 hours, still more typicallyfrom 6 hours to 12 hours and yet more typically about 10 hours. Theprecursor of the laser output power enhancing substance or gaseoushalogen-containing reagent in this embodiment is typically a halogen ora hydrogen halide, more typically chlorine, bromine, hydrogen bromide orhydrogen chloride, and the inert gas during the pre-conditioning periodis typically neon, argon or helium, more typically neon. Theconcentration of precursor of the laser output power enhancing substanceor gaseous halogen-containing reagent in the inert gas in thepre-conditioning period is typically from 0.01% to 100% by volume, moretypically from 2% to 40% by volume, even more typically from 3% to 30%,still more typically from 4% to 25%, yet more typically from 5% to 20%,from 8% to 15% or about 10% by volume.

Usually, during the pre-conditioning period, the partial pressure of theprecursor of the laser output power enhancing substance or gaseoushalogen-containing reagent is in the range of 13 Pa to 101 kPa, moretypically 100 Pa to 50 kPa, or 250 Pa to 40 kPa or 500 Pa to 30 kPa or 1kPa to 25 kPa or 2 kPa to 20 kPa or 4 kPa to 18 kPa or 6 kPa to 16 kPaor 8 kPa to 15 kPa or 10 kPa to 14 kPa, still more typically about 13kPa. During the pre-conditioning period, the partial pressure of theinert gas is usually in the range of 0 to 13 kPa, more typically 1 kPato 12 kPa, or 1.5 kPa to 11 kPa or 2 kPa to 10 kPa or 2.5 kPa to 9 kPaor 3 kPa to 8 kPa or 3.5 kPa to 7 kPa or 4 kPa to 6 kPa, still moretypically about 5.3 kPa.

Typically, in this embodiment, after the pre-conditioning period themetal vapour laser is operated with a buffer gas including an additiveas described above, the additive more typically being hydrogen. Underthese conditions, the properties of the plasma in the discharge tube maybe finely controlled by the partial pressure of the additive in thebuffer gas. In particular, the rate at which hydrogen halide is producedin the discharge tube from metal halide formed as described above can beaffected by varying the partial pressure of hydrogen or other similaradditive in the buffer gas. Alternatively, the concentration of hydrogenhalide can be controlled by varying the flow rate of a buffer gasmixture which is a mixture of hydrogen or other similar additive and aninert gas such as neon. The partial pressure of additive included in thebuffer gas in this and other embodiments of the invention is dependenton the residual level of laser output power enhancing substance in thedischarge tube (which varies over time during the operation of thelaser) and the application for which the laser is required (for examplemaximum plane/plane output power or maximum high beam, quality outputpower). Typically, the range of partial pressures of additive in thebuffer gas after the pre-conditioning phase is from 1 Pa to about 2000Pa, more usually from 2 Pa to 1500 Pa or 3 Pa to 1200 Pa or 4 Pa to 1000Pa or 5 Pa to 950 Pa or 6 Pa to 900 Pa or 7 Pa to 850 Pa or 8 Pa to 800Pa or 10 Pa to 750 Pa or 12 Pa to 700 Pa, even more usually about 13 Pato about 665 Pa.

In one particular embodiment of the second form of the invention, theprocess involves pre-conditioning the discharge tube with eitherhydrogen halide or halogen. The optimal concentration of hydrogen halideproduced in the tube during lasing conditions may be controlled byvarying the level of hydrogen (or D₂) added to a neon buffer gas.Alternatively the concentration of H₂ in the plasma region may becontrolled by a “getter” as exemplified above. As a further alternative,a quantity of metal hydride may be included in the discharge tube. Whenthe discharge tube is brought to operating temperature the hydridedissociates into metal and hydrogen atoms, and the quantity of additive(hydrogen atoms in this case) may be controlled, at least in part, byvarying the temperature of the laser tube. Typically, however, somehydrogen gas is also included in the buffer gas of the laser in thisform of the invention. The concentration of H₂ introduced into a neon(or helium) buffer gas is dependent on the residual level of halogenatoms in the tube (which may vary with time) and the application forwhich the laser is required (i.e. maximum plane/plane output power ormaximum high beam, quality output power), and may be between from 13 Pato 665 Pa. Indeed, the laser can still be operated in the same way as aconventional metal vapour laser (after pre-conditioning) by flowing apure inert gas such as neon, as buffer gas. The laser may also beoperated by flowing a pure inert gas as buffer gas through the dischargetube when the discharge tube is provided with an amount of a metalhalide such as TaCl₅, as described herein.

In another embodiment of the invention, the buffer gas is admitted tothe metal vapour laser throughout the operation of the laser, as apremixture of the inert gas and the laser output power enhancingsubstance or its precursor. Alternatively, in this embodiment the inertgas and the laser output power enhancing substance or its precursor maybe separately admitted to the discharge tube throughout the operation ofthe laser. For example, hydrogen halide may be added to the buffer gasprior to its admission to the discharge tube, or an inert gas such asneon and hydrogen halide may be separately admitted to the dischargetube. In this embodiment, the concentration of laser output powerenhancing substance of its precursor in the discharge tube during theoperation of the metal vapour laser is typically 0.1 Pa to 1000 Pa, moretypically from 0.5 Pa to 800 Pa or 1 Pa to 600 Pa or 2 Pa to 500 Pa or 4Pa to 400 Pa or 6 Pa to 350 Pa or 8 Pa to 300 Pa or 10 Pa to 280 Pa,still more typically from 13 Pa to 260 Pa.

In a still further particular embodiment of the invention theconcentration of hydrogen halide in the plasma region can be controlledby adding a combination of H₂ (or D₂ or another isotope of hydrogen) andhydrogen halide, or a combination of H₂ (or D₂ or another isotope ofhydrogen) and halogen to the buffer gas.

In each embodiment of the process of the invention, the metal vapourlaser may be operated with a slow flow of the buffer gas maintainedthrough the metal vapour laser during its entire operation, or it may beoperated without the buffer gas flowing. When the metal vapour laser isoperated without buffer gas flowing, it will be referred to herein asoperating in “sealed off” mode; however it will be appreciated that inthe “sealed off” mode it is not essential that the inlet and outlet forthe buffer gas to the metal vapour laser be physically sealed, althoughthey may be. In the embodiment of the invention in which the laserdischarge tube is pre-conditioned with a precursor of a laser outputpower enhancing substance, the metal vapour laser may be operated in“sealed off” mode after the pre-conditioning period has been completed,and stable operation of the laser has been achieved. It has been foundthat once stable optimum or near-optimum conditions for the metal vapourlaser of the invention have been achieved, the laser may be operated forextended periods (for example at least 50-100 hours) in the “sealed off”mode. This capability provides a substantial advantage of the metalvapour laser of the invention compared to some previously known metallasers.

In all embodiments of the invention operation of the metal vapour laseris monitored by measuring the voltage and current pulses supplied to thelaser, or by measuring the laser power. A low peak current pulse duringoperation is indicative of excessive levels of laser output powerenhancing substance, which may be controlled by decreasing an amount ofadditive such as H₂ in the buffer gas, or decreasing the amount of laseroutput power enhancing substance added to the buffer gas, or reducingthe operating temperature of the laser (depending on the form of theinvention being used) or a combination of two or more of these actions;and a high peak current pulse indicates low levels of laser output powerenhancing substance, which may be controlled by taking the oppositeaction or actions.

When the metal vapour laser is operated with a flow of buffer gas, theflow rate is typically from 0.1 to 200 atm.mL/min per L, more typicallyfrom 1 to 200, 2 to 150, 3 to 100, 4 to 80, 5 to 70, 6 to 60, 7 to 55, 8to 50, 9 to 45 or 10 to 50 atm.mL/min per L of active volume of thedischarge tube of the metal vapour laser.

The metal vapour laser of the present invention is typically capable ofproducing at least 10%, more typically at least 15%, 20%, 30%, 40%, 50%,60%, 70%, 80% or 90% higher power output, even more typically at least100%, 120%, 140%, 160%, 180%, 200%, 250% or 300% higher power output,compared to previously known metal vapour lasers. The efficiency ofoperation of the metal vapour laser of the present invention issimilarly at least 10%, more typically at least 15%, 20%, 30%, 40% 50%,60%, 70%, 80% or 90% higher than that of previously known metal vapourlasers, even more typically the efficiency of operation of the metalvapour laser of the present invention is at least 100%, 120%, 140%,160%, 180%, 200%, 250% or 300% higher than that of previously knownmetal vapour lasers.

The process of the present invention permits operation of the metalvapour laser over a wide range of operating parameters such asexcitation circuit configurations, repetition rates, buffer gaspressure, laser aperture size, buffer gas flow rates etc.

Furthermore, in the metal vapour laser of the present invention a highfraction of the output power is typically available with high beamquality. This is important in many applications of metal vapour laserscurrently under development, such as frequency conversion to theultraviolet, for ultraviolet micromachining of polymers, ceramics andother materials, pumping of dye lasers, pumping of tunable solid statelasers, industrial plastochemistry, non-linear frequency conversion,medical uses, etc. A metal vapour laser of the present invention isuseful in any of these applications, and in other known applications ofmetal vapour lasers, such as described, for example, in Hecht, J., TheLaser Guidebook, Second Edition, McGraw-Hill, Inc., 1992 at pages207-210, the contents of which are incorporated herein by reference.Using a metal vapour laser of the present invention in non-optimisedultraviolet generation, ultraviolet powers of over 3 W at overall energyconversion efficiencies of about 0.07% have been achieved. Furthermore,these advantages are obtainable, with the metal vapour laser of thepresent invention, by utilising existing discharge tube technology toachieve characteristics similar to or better than those obtainable frompreviously known metal vapour lasers.

The mechanism by which the inclusion of laser output power enhancingsubstance in a metal vapour laser provides a greater power output fromthe laser is not known with certainty, but, without wishing to be boundby any theory, the inventors speculate that the laser output powerenhancing substance, for example hydrogen halide, reduces the prepulseelectron density in the laser discharge tube during the interpulseperiod via a mechanism of dissociative electron attachment, this in turnleading to improved prepulse conditions and improved matching betweenthe discharge tube and the excitation circuit, and consequently improvedoutput power. It is observed that when both very low and high amounts oflaser output power enhancing substance are utilised in the metal vapourlaser of the invention, there may be little or no increase in the poweroutput of the laser. However, with intermediate amounts, an optimumpower output occurs. Such an optimum may be readily determined bypersons of ordinary skill in the relevant art using no more than routineexperimentation, given the teaching provided herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates in diagrammatic longitudinal cross-section a laserdischarge tube assembly for a metal vapour laser of the presentinvention.

FIG. 1B is a transverse cross-section through the laser discharge tubeassembly illustrated in FIG. 1A, at plane A—A.

FIG. 2 is a diagrammatic longitudinal cross-section of an end piece ofthe laser discharge tube assembly illustrated in FIG. 1A.

FIG. 3 illustrates in diagrammatic form a vacuum and gas handling systemfor the laser discharge tube assembly illustrated in FIG. 1A.

FIG. 4A illustrates an excitation circuit to provide excitation pulsesto the laser discharge tube assembly illustrated in FIG. 1A.

FIG. 4B illustrates an alternative excitation circuit, including threestage magnetic pulse compression.

FIGS. 5A to 5C are graphs illustrating the effect of various additivesin the buffer gas of a metal vapour laser on the output power of thelaser.

FIGS. 6A to 6C illustrate the spatial near-field profile of the laserbeam of a metal vapour laser of the invention, when operated withhydrogen chloride added to the buffer gas, the concentration of hydrogenchloride varying from 0% (FIG. 6A) to 1% by volume (FIG. 6B) and 2% byvolume (FIG. 6C).

FIG. 7 is a graph of the temporal evolution of the laser pulse of ametal vapour laser of the invention, when operated with varyingconcentrations of hydrogen chloride added to the buffer gas.

FIGS. 8A and 8B are graphs showing the peak amplitude of the excitationcurrent and voltage pulses, respectively, at a range of hydrogenchloride concentrations in the buffer gas of a metal vapour laser of theinvention.

FIGS. 9A and 9C are graphs illustrating the relationship between pulserepetition rates and power output of lasers according to the invention,using three different buffer gas mixtures.

FIG. 10 is a schematic representation of a cross-section through one endof a laser discharge tube assembly in accordance with the presentinvention, illustrating some possible positions where a quantity of asecond metal, salt or halide thereof, or of a metal hydride, may belocated.

BEST MODE AND OTHER MODES FOR CARRYING OUT THE INVENTION

FIG. 1A illustrates in diagrammatic longitudinal cross-section a laserdischarge tube assembly for a medium scale metal vapour laser (nominally25 W output power) of the present invention. FIG. 1B is a transversecross-section through the laser discharge tube assembly illustrated inFIG. 1A, at plane A—A. As seen in FIGS. 1A and 1B, laser discharge tubeassembly 1 incorporates ceramic discharge tube 10 surrounded by fibrousalumina ceramic high temperature insulator 20 which is located in silicavacuum tube 30. Discharge tube 10 is constructed of alumina(Haldenwanger Alsint nominal purity greater than 99.5%) and hasdimensions of 25.5 mm internal diameter, 3 mm thickness and 1000 mmlength, giving an active volume of about 0.5 L. The ends of vacuum tube30 are sealed to end pieces 40, 41 (described in more detail below)which support anode and cathode 50, 51 respectively.

Silca vacuum tube 30 is surrounded by a thin layer of insulation 101(Triton Kaowool) which is itself surrounded by heating element 100 toprovide supplementary heating and another thicker layer of insulation102 (Triton Kaowool). The surface of outer layer of insulation 102 issurrounded by aluminum foil shell 105. Heating element 100 compriseslengths of nichrome wire (1.7 Ω/m) threaded through a thin ceramic tube(not shown) and is capable of heating discharge tube 10 to a maximum ofabout 950° C. The assembly of heater 100, insulator layers 101, 102 andfoil shell 105 is surrounded by an air space about 1 cm thick within theco-axial current return 108 (seen in FIG. 1B) of the laser. Dischargetube 10 is transversely air cooled through this space, by air providedfrom fans (not shown) mounted below laser tube assembly 1.

Discharge tube assembly 1 as illustrated is provided with a quantity ofpieces of pure tantalum metal 130 in the region between cathode 51 andinsulator 20. However it will be appreciated that tantalum pieces 130may be placed at other locations in laser discharge tube assembly 1 asillustrated in FIG. 10 described below, or they may be omitted.

Vacuum tube 30 is provided, adjacent each end piece 40, 41, with gasoutlet 90 and gas inlet 95 for connection to a vacuum and gas handlingsystem, as illustrated in FIG. 3 described below.

Discharge tube 10 is loaded periodically (approximately each 200 hoursof laser operation) with from about 10 to 15 g of high purity metalpieces, typically copper pieces, 110.

Referring to FIG. 10 there is illustrated a schematic representation ofa cross-section through one end of a laser discharge tube assembly inaccordance with the present invention, illustrating some possiblepositions where a quantity of a second metal (other than the first metalwhich is the lasing metal), salt or halide thereof, or of a metalhydride, may be located. In FIG. 10, end of laser discharge tubeassembly 600 comprises discharge tube 610 surrounded by insulator 620within silica vacuum tube 630. One end of vacuum tube 630 is closed withend piece assembly 641 which includes electrode 651 and window 661,which is sealed against the exterior of end-piece assembly 641 by O-ring663. End-piece assembly 641 is also provided with gas input line 690.Positions at which a quantity of metal, metal halide, other metal salt,or metal hydride may be positioned are as follows:

671: under electrode 651;

672: near window 661;

673: in gas input line 690, for example in a temperature controlled oven(not shown);

674: impregnated in insulator 620; or

675: as electrode 651, or forming part of electrode 651, or impregnatedin electrode 651.

FIG. 2 is a diagrammatic longitudinal cross-section of end piece 40 oflaser discharge tube assembly 1 illustrated in FIG. 1A, which containselectrode 50 and laser window 60 and provides a vacuum seal to silicavacuum tube 30. End piece 41 (seen in FIG. 1A) is of identicalconstruction to end piece 40. Anode 50, made from rolled tantalum sheet,is mounted on removable copper supports 55 for quick replacement, andheld in place by a pressure fit. Anode 50 may also be made, for example,from nickel or stainless steel. Window 60 consists of a 50 mm diametersilica flat polished to λ/10 tolerance. Neoprene O-ring 65 forms avacuum seal between window 60 and the body of end piece 40. Window 60 isheld in place against O-ring 65 by virtue of the vacuum in silica vacuumtube 30. Viton O-ring 70 forms a vacuum seal between silica vacuum tube30 (see also FIG. 1A) and the body of end piece 40. Cooling of end piece40 is provided by an oil-filled section, air cooling fins 92 andrecirculated deionised water which enters end piece 40 at a port on thetop surface (not shown) and exits at a port on the bottom surface (notshown). The oil-filled cooling section consists of a coolant jacket 91sealed by O-ring 93 to the body of end piece 40, jacket 91 definingtherewith in coolant chamber 90 which is filled with oil. Coolant jacket91 is secured to end piece 40 by means of bolts 94.

FIG. 3 illustrates in diagrammatic form a vacuum and gas handling systemfor a metal vapour laser of the invention. Vacuum/gas handing system 200comprises a gas mixing section and a vacuum pumping section.

Referring to FIG. 3, the vacuum pumping section of vacuum/gas handlingsystem 200 consists of turbomolecular pump 210 coupled via zeolite watertrap 215 to back-up rotary pump 220. Zeolite water trap 215 also servesto prevent pump oil from rotary pump 220 contaminating the vacuumsystem. The vacuum pumping system communicates via manifold 265,ultra-high vacuum (UHV) valve 270 and leak valve 275 (arranged inparallel) with gas outlet 90 of laser discharge tube assembly 1 (seealso FIG. 1A). Gas outlet 90 is fitted with capacitance manometer vacuumgauge 280.

The gas mixing system permits any one or more of three ultra-high puritygas cylinders 230, 231, 232 to be on-line at any one time. Further gascylinders (not shown) may be connected to the system if desired.Cylinders 230, 231, 232 are connected to mixing chamber 240 fitted withcapacitance manometer vacuum gauge 245. Mixing chamber 240 communicatesvia leak valve 250 with gas inlet 95 of plasma discharge tube assembly 1(see also FIG. 1A). Mixing chamber 240 may also be connected to thevacuum pumping section by means of UHV valve 255. A further point ofconnection of a fourth gas cylinder is provided on the open side ofsecond UHV valve 260, which otherwise serves as an air admission portwhen the laser is not in use.

Each fitting is sealed to the remainder of the system via Conflatflanges (not shown).

FIG. 4A illustrates an excitation circuit which may be used to provideexcitation pulses to the laser discharge tube assembly illustrated inFIG. 1A.

Referring to FIG. 4A, excitation circuit 300 is a thyratron-switchedpulse charging circuit consisting of a 0-9 kV DC supply (not shown)connected to terminals 305, 306. The DC supply is connected to storagecapacitor 310 (2-6 nF) via saturable inductor 320, magnetic assist 321,resistor 325 (100 Ω) and diode 330 (Varo VC80X). Diode 350 and seriesresistor 355 (1 kΩ) are provided to prevent the reverse voltages whicharise from ringing in the discharge circuit from appearing acrossthyratron 340. Resistor 360 (100 Ω wirewound) and inductor 365 tie oneside of storage capacitor 310 to ground during the charging of storagecapacitor 310. Peaking capacitor 370 (0.8-2 nF) is connected acrosselectrodes 50, 51 of laser discharge tube assembly 1 (see FIG. 1A andFIG. 2).

When the laser is operated, the temperature of discharge tube 10 isbrought to about 850° C. by passing current through heating element 100.If it is desired to pre-condition discharge tube 10, at this stage amixture of neon and hydrogen chloride (partial pressure of hydrogenchloride of about 13.3 kPa) is admitted from mixing chamber 240 (seeFIG. 3) into discharge tube 10 via gas inlet 95 and is allowed to flowslowly through discharge tube 10 and exit via gas outlet 90, for aperiod of about ten hours. At the end of this period, or if dischargetube has not been pre-conditioned, discharge tube 10 is evacuated byoperating rotary pump 220 and, when a sufficiently low pressure has beenreached, turbomolecular pump 210. Discharge tube 10 is then filled witha pure Ne buffer gas. With external heater 100 still on, about 1.5 kWinput power is applied to terminals 305, 306 of excitation circuit 300until the lasing threshold temperature (typically 1400° C. in a coppervapour laser) has been reached in discharge tube 10. External heater 100is then turned off and the input power at terminals 305, 306 isincreased to about 3 kW. At this point, if no pre-conditioning ofdischarge tube was carried out, a mixture of hydrogen gas and hydrogenchloride is added to the neon buffer gas flowing through discharge tube10. Alternatively, if discharge tube 10 has been pre-conditioned, aquantity of hydrogen gas is mixed with the neon buffer gas. The outputpower of the laser is then adjusted to its optimum by adjustment of thepartial pressure of hydrogen in the buffer gas. The laser typicallyreaches a steady state after about 1 hour, and at that time the flow ofbuffer gas may be stopped.

Excitation circuit 300 operates as follows. After each discharge pulsethe high voltage DC power supply applied to terminals 305, 306resonantly charges storage capacitor 310 to nearly twice the supplyvoltage through resistor 325, diode 330 and saturable inductor 320.Saturable inductor 320 provides extended hold off (˜20 μs) to thyratron340 during its recovery stage after each pulse.

When thyratron 340 is triggered, charge is rapidly transferred fromstorage capacitor 310 to peaking capacitor 370 through thyratron 340until the peaking capacitor voltage is sufficiently high to causebreakdown in discharge tube 10. At this time both storage capacitor 310and peaking capacitor 370 are rapidly discharged through discharge tube10. In order to ensure the fastest possible rise time of the dischargecurrent pulse, the inductance of the peaking-capacitor—discharge-tubecircuit is minimised by maintaining a coaxial geometry throughout.During the fast switching stage, the inductance of wire-wound resistor350 is sufficiently high that peaking capacitor 370 is dischargedthrough discharge tube 10 rather than through resistor 360.

Additional thyratron protection is provided by magnetic assist 321 tohold off the current flowing through thyratron 340 for severalnanoseconds during the switching period in which thyratron 340 is goinginto conduction. Magnetic assist 321 is provided by including abouttwenty ferrite toroids on the line connecting the anode of thyratron 340to storage capacitor 310, providing a saturable inductance in thestorage capacitor—thyratron—peaking capacitor charge transfer loop.

Alternatively, magnetic switching techniques may be employed to provideexcitation pulses to the laser discharge tube. One suitable circuitarrangement is illustrated in FIG. 4B, which represents a magneticallyassisted L-C inverter followed by three stage magnetic pulsecompression. Circuit 400 illustrated in FIG. 4B operates as follows:

Storage capacitors 410 and 411 are resonantly charged (in parallel) toapproximately twice the supply voltage (V₀) at terminals 415, 416 viacharging inductor 420, charging diode 425 and tube bypass inductor 428which is connected across the electrodes of metal vapour laser dischargetube 470. When thyratron 430 is switched, the voltage on storagecapacitor 411 is reversed via ringing in the subcircuit comprisingtransfer inductor 435 and storage capacitor 411. The value of transferinductor 435 is chosen so that the time taken for the voltage on storagecapacitor 411 to swing from +2 V₀ to −2 V₀ is equal to the hold-off timeof first saturable inductor 440. Just prior to the saturation of thefirst magnetic pulse compression stage 440, the voltage across thecombination of storage capacitors 410, 411 reaches about 4 V₀. As thefirst saturable inductor 440 saturates, the charge on storage capacitors410, 411 is transferred to capacitor 445, in a time that is shorter (bya factor called the compression ratio) than the time taken to initiallyinvert the voltage on storage capacitor 411. As the charge transfer tocapacitor 445 nears completion, second stage compressor 450 saturates,and the charge from capacitor 445 is transferred from capacitor 445 tocapacitor 455 in time again shorter by the compression of this stage,than capacitor 455 was charged. The transfer from capacitor 445 is ofthis stage, than capacitor 455 was charged. The transfer from capacitor455 to capacitor 465 occurs in the same fashion, caused by saturation ofthird stage compressor 460, with further compression of the chargetransfer time (and hence voltage rise time). As the voltage on capacitor465 rapidly rises (typically ten times faster than the voltage inversionon storage capacitor 411 occurs) current begins to flow throughdischarge tube 470 and laser action is excited.

Because of the impedance mismatch between discharge tube 470 and excitorcircuit 400, generally some of the energy that is switched to thedischarge tube via capacitor 465 is reflected. This reflected energy cancause extreme stress on the switching element, and circuit 400incorporates two techniques for minimising this stress. Magnetic assist480 delays zero-current crossing during grid-anode recovery untilthyratron 430 has recovered. Snubber 490 (consisting of high-powerresistor 491 and diode 492 in series) partially adsorbs energy that isreflected from discharge tube 470.

Other forms of magnetic pulse compression exciters that are routinelyused with metal vapour lasers may be used for the metal vapour laser ofthe present invention. For example, circuits incorporating pulsetransformers to increase the voltage from a low voltage source to levelsappropriate for laser excitation may be used. Circuits using differentswitching elements other than thyratrons (for example solid-stateswitches) may also be used, and capacitance transfer topologies can beused instead of L-C inverter circuits. Such circuits are well known foruse with metal vapour lasers and their application with the metal vapourlaser of the present invention lies well within the capability ofpersons of ordinary skill in the relevant art.

The advantages provided by the metal vapour laser of the presentinvention are not dependent on the type of excitation used for the laserdischarge tube.

EXAMPLES Example 1

Operational characteristics of a metal vapour laser of the inventionwithout pre-conditioning

A copper vapour laser having a 25 mm diameter by 1000 mm long laserdischarge tube was operated with a neon buffer gas flowing at ˜3atm.mL/min, including various added gases, at 17.5 kHz pulse repetitionrate unless otherwise stated, with 1.5 nF of storage capacitance and 0.6nF of peaking capacitance.

FIGS. 5A to 5C illustrate the effect of the added gases employed on theoutput power of the laser. It will be seen that in the exampleillustrated in FIG. 5A the inclusion of chlorine and bromine resulted ina decrease i the output power over the range of concentrations tested,but the addition of hydrogen chloride and hydrogen bromide resulted in asubstantial increase in output power over a range of concentrations,with an optimum at 1-2% hydrogen chloride or hydrogen bromide in thebuffer gas by volume.

FIG. 5B illustrates the effect of added hydrogen in the buffer gas onthe output power of the laser. In this example, various amounts byvolume of HBr, HCl, 1:1 H₂—Br₂ and 1:1 H₂/Cl₂ were added to the buffergas. It will be seen that under these conditions, the addition of amixture of hydrogen and bromine enhanced the peak output power of thelaser compared to the addition of the same amount of HBr, whereas theaddition of a mixture of hydrogen and chlorine reduced the peak outputpower of the laser compared to the addition of the same amount of HCl.

FIG. 5C illustrates the effect of buffer gas flow rate on peak outputpower of the laser, with various amounts of HCl and HBr added to theneon buffer gas. In this experiment, the laser was operated at slow(approx. 4-5 atm.mL/min) or fast (approx. 60 atm.mL/min) flow rates. Theperformance of the laser was best when operating with added HBr atrelatively fast buffer flow rate, whereas at more typical buffer flowgas rates of about 4 atm.mL/min, optimal performance is achieved whenemploying HCl/neon buffer gas mixtures.

FIGS. 6A to 6C illustrate the spatial near-field profile of the laserbeam under the conditions described above in this Example, with hydrogenchloride added to the buffer gas, when the concentration of hydrogenchloride was varied from 0% (FIG. 5A) to 1% by volume (FIG. 6B) and to2% by volume (FIG. 6C). Similarly, FIG. 7 provides a graph of thetemporal evolution of the laser pulse, and FIGS. 8A and 8B plot the peakamplitude of the excitation current and voltage pulses, respectively, ata range of hydrogen chloride additive concentrations. It will be seenthat particularly in the range of 1-2% by volume added HCl, the additionof hydrogen chloride to the buffer gas significantly modified thespatio-temporal evolution of the laser pulse and altered the peakamplitude of the excitation voltage and current pulses.

Example 2

Operational characteristics of a copper vapour laser of the inventionwith a buffer gas including mixtures of hydrogen and hydrogen halides

The copper vapour laser used in these experiments was a 40 mm×1.5 m longdevice (nominally a 55 W device when operated at 4.5 kHz) employing athree stage magnetic pulse compression excitation circuit. For thecourse of these experiments the laser was operated at a pulse repetitionfrequency of 9 kHz at reduced input powers (˜80% of optimum) so thatpotential overheating effects, due to the improved impedance matchingresulting from some buffer gas additives, could be avoided. The buffergas flow rates were 2-5 atm.mL/min.

Table 1 shows the output power produced by the laser when operating witha pure neon buffer gas, a 2% H₂ additive, and a range of amounts of HBrwith 1% H₂ buffer gas additive. All additive amounts are given inpercentages by volume. Table 2 shows the output power when operatingwith a pure neon buffer gas, a 2% H₂ additive, and a range of amounts ofHCl with 1% H₂ buffer gas additive.

TABLE 1 % HBr % H₂ Output Power (W) 0 0 20 0 2 34   0.5 1 38 1 1 42 2 145 3 1 46

TABLE 2 % HCl % H₂ Output Power (W) 0 0 20 0 2 34   0.2 1 44   0.5 1 501 1 40

Significant increases in the output power were observed when employingcombinations of HBr and H₂, the maximum output power corresponding tothe addition of 2-3% HBr to a 1% H₂—Ne buffer gas. The laser outputpower under these conditions is greater than that observed whenemploying 2% H₂—Ne buffer gas mixtures. However, the best results wereachieved when employing combinations of HCl and H₂—Ne buffer gasmixtures. A maximum of 50 W was observed when employing a 0.5% HCl-1%H₂—Ne buffer gas.

Example 3

Operational characteristic of a metal vapour laser of the invention witha pre-conditioning period

In this Example, a 25 mm diameter copper vapour laser discharge tube waspre-conditioned at 850° C. with the laser off by including 13 Pa to 101kPa (typically about 13 kPa) partial pressures of HCl, HBr, Cl₂ and Br₂in a neon buffer gas, typically at a partial pressure of from 1.3 kPa to13 kPa, most typically at a partial pressure of about 5.3 kPa, for from30 minutes to several hours. Subsequently, the laser operated under theconditions described in Example 1, with the exception that a mixture ofhydrogen-neon (about 13-260 Pa partial pressure of hydrogen) and pureneon were alternately flowed through the tube until the voltage/currentcharacteristics of the laser resembled those shown in FIGS. 8A and 8B.Under these conditions, the power output of the laser increased from 10to 30 W for approximately 3 kW input power and was very stable. It wasfound that the best results were obtained in this Example when thedischarge tube was pre-conditioned with either HCl or Cl₂.

Table 3 below shows the maximum total output power which achieved in aseries of trials with a plane/plane resonator in a 25 mm diameter coppervapour laser of the invention having a hydrogen chloride/hydrogenmixture included in the buffer gas (about 13 Pa to 260 Pa partialpressure of each), together with the non-ASE high beam quality outputpowers produced from a 25 mm diameter copper vapour laser of theinvention when employing on-axis unstable resonators. For comparison,corresponding output power values obtained when the same laser isoperated under the same conditions but without added hydrogen chloride(pure neon buffer gas or neon-hydrogen buffer gas: about 13-260 Papartial pressure of hydrogen) are also provided.

TABLE 3 Total output power (W) from a 25 mm copper vapour laser Pulserepetition Buffer gas frequency (kHz) Pure neon Neon-hydrogen H₂—HCl—NeUSR (M = 100)  4-5 8.1 8.3  8.95  9-10 5.0 10.6  23.7 18-20 1.6 8.1 20.825 — — 20.0 Plane/Plane 27 <20 — 50.6

Table 4 provides similar results for a 40 mm diameter copper vapourlaser, which had been pre-conditioned with HCl as described in Example 2prior to operation with a neon-hydrogen buffer gas mixture.

TABLE 4 Total output power (W) from a 40 mm copper vapour laser Pulserepetition Buffer gas frequency Ne/H₂; laser tube pre- (kHz) Pure neonNeon-hydrogen conditioned with HCl USR 4 14.8 (M = 20) 27.6 (M = 20) —12 ˜10 (M = 125) — 67 (M = 125) Plane/Plane 4 46.3 56.4 (69 peak) — 552.8 63.8 ˜75 13 ˜30 — 100.6

Examples 4 and 5

Operational characteristics of a copper vapour laser of the invention,including tantalum metal, with a pre-conditioning period

Example 4

In this example, the laser described in Example 1 was provided withabout 25 g of tantalum metal pieces between the cathode and the plasmatube, and was pre-conditioned for 3-24 hours with about 13 kPa of HCl ata laser wall temperature of about 1000° C. before being evacuated andoperated with various buffer gases.

FIG. 9A shows the total output power of the laser operating with aplane/plane resonator with pure Ne, H₂—Ne (2% hydrogen by volume) andHCl—H₂—Ne (0.5% HCl and 1% H₂ by volume) buffer gas mixtures, over arange of pulse repetition frequencies (PRFs). This laser produced amaximum of ˜20 W when operated with a 2% H₂—Ne buffer gas mixture at aPRF of 17 kHz. The output power of the laser operating with the H₂—Nebuffer gas mixture decreased when the PRF was elevated above 17 kHz.Laser performance improved significantly when the H₂—Ne buffer gasmixture was replaced with the H₂—HCl—Ne mixture. In this case the laserproduced 32 W at a PRF of 18 kHz. Furthermore, the H₂—HCl—Ne buffer gasmixture permitted efficient PRF scaling of this device. At PRFs of 25and 29 kHz the laser produced 40 W and 51 W respectively (compared to<<20 W when employing conventional buffer gases).

FIG. 9B shows the high beam quality output power of the laser operatingwith a high magnification (M=100) on-axis unstable resonator and pureneon buffer gas, the H₂-neon buffer gas mixture or the HCl—H₂-neonbuffer gas mixture over a range of PRFs. The high beam quality (HBQ)output power extraction from this device was unaffected by the buffergas composition when operated at low PRF (˜4 kHz). However, significantimprovements in the HBQ output were observed when the laser was operatedat elevated PRFs with either H₂ or H₂—HCl added to the neon buffer gas.Added H₂ increased the HBQ output power of the laser by up to 3-4 times,while added H₂—HCl increased the output power by up to 6-10 times,leading to a maximum of 24 W of high beam quality output.

Example 5

In this example, the laser described in Example 2 was provided withabout 25 g of tantalum metal pieces between the cathode and the plasmatube, and was pre-conditioned for 3-24 hours with about 13 kPa of HCl ata laser wall temperature of about 1000° C. before being evacuated andoperated with various buffer gases.

FIG. 9C shows the total output power of the laser operating with aplane/plane resonator with pure Ne, H₂—Ne (1% hydrogen by volume) andHCl—H₂—Ne (0.5% HCl and 1% H₂ by volume) buffer gas mixtures, over arange of PRFs. The laser produced a maximum of 53 W at a PRF of 4.3 kHzwhen operated with pure Ne buffer gas, and a maximum of 65 W whenoperated with a 1% H₂—Ne mixture. Output power decreased when the laserwas operated with conventional buffer gas mixtures at elevated PRFs, butusing the HCl—H₂—Ne buffer gas mixture the laser produced 80 W at a PRFof 4.3 kHz (compared to 65 W when operating with H₂ additive alone) anda maximum of 101 W when operated at an elevated PRF of 12 kHz (comparedto ˜30 W when operating with added H₂ only.) These output powers arecomparable to those normally produced by copper vapour laser of twicethe volume (ie. diameter>60 cm and lengths >2 m).

When the laser described in this Example was operated with theHCl—H₂-neon buffer gas mixture, similar improvements to those describedin Example 4 were observed in the extraction off non-ASE output power.At a PRF of 6 kHz the laser produced 15 W of high beam quality outputwhen operating with a high magnification (M=125) on-axis unstableresonator and pure neon buffer gas, and 53 W with the HCl—H₂-neon buffergas mixture. At a PRF of 12 kHz this laser only produced 4 W whenemploying pure neon buffer gas, which increased by a factor of >15 to 67W when operating with the HCl—H₂-neon buffer gas mixture. This laseralso produced 15 W of high beam quality output when operating with theHCl—H₂-neon buffer gas mixture and with a self filtering unstableresonator.

The specific output powers of the 25 mm and 40 mm diameter lasersdescribed in Examples 4 and 5, namely 104 mW/cm³ and 54 mW/cm³respectively, are the highest yet achieved from copper vapour lasers ofthis size and are comparable to those obtained from copper HyBrID(“Hydrogen Bromide In Discharge”) lasers of similar active volumes.

The improved output power extraction of copper vapour lasers operatingat elevated PRFs with H₂-HCl—Ne buffer gas mixtures results from bothmodified temporal and spatial gain characteristics. The duration of acopper vapour laser output pulse is significantly shortened (for thesame peak power) as the PRF is elevated, leading to a reduction in totaloutput power. For example, at a PRF of 17 kHz the pulse duration of the25 mm diameter copper vapour laser operating with a pure Ne buffer gaswas only ˜30 ns (compared to 55 ns at a PRF of 4.5 kHz) or only tworound-trips through the resonator. This characteristic limits theusefulness of copper vapour lasers when required for high repetitionrate, high beam quality applications. A hydrogen buffer gas additiveextends the efficient PRF scaling capability of the 25 mm and 40 mmdiameter copper vapour lasers to 17 and 6 kHz respectively. The H₂—HCladditive increases the pulse duration by a greater amount than added H₂.Indeed, at a PRF of 17 kHz the pulse duration of the 25 mm diametercopper vapour laser was increased from 30 ns to ˜60 ns, consistent withradiation undergoing an additional two round-trips through the gainregion. The output power of the 25 mm and 40 mm diameter copper vapourlasers with included tantalum metal and H₂—HCl—Ne buffer gas mixturescaled linearly up to the maximum PRFs (ie. 30 and 12 kHz respectively)available with the existing laser excitation circuits.

The modified spatial characteristics also contribute to the improved PRFscaling capability of copper vapour lasers operating with H₂—HCl—Nebuffer gas. At low PRFs the radial intensity profile of copper vapourlasers operating with pure Ne approximates a “top-hat” structure.However, as the PRF is elevated (or the aperture scaled) the intensityprofile becomes increasingly annular. At PRFs>20 kHz the 25 mm diametercopper vapour laser output is visibly restricted to a ring near the tubewall. At PRFs of ˜20 kHz the intensity profile of the 25 mm diametercopper vapour laser operating with H₂—Ne buffer gases is stillrelatively annular. By comparison, the same copper vapour laseroperating with H₂—HCl added to the neon buffer gas has an axially peakedintensity profile at PRFs up to and beyond 20 kHz.

Example 6

Operational characteristics of a copper vapour laser of the invention,including tantalum pentachloride

Enhanced performance of the 40 mm diameter laser described in Example 2has also been observed using metal halide as the sole source of halogen.The laser yielded about 75 W of output power when operated with a neonbuffer gas containing 2% by volume of hydrogen, withoutpre-conditioning, but with a small amount of TaCl₅ powder placed in atray underneath the cathode, using either copper or stainless steelelectrodes. Similar increases in output power were also observed whenthe halogen source comprised a piece of insulator impregnated with traceamounts of TaCl₅ powder or a section of glass matting impregnated withtrace amounts of TaCl₅. The output power of the laser increased stillfurther to 92 W when the laser was operated under these same conditionsbut with electrodes manufactured from tantalum.

It will be seen that output powers achievable by a metal vapour laser ofthe present invention represent a substantial increase over thoseachievable by conventional metal vapour lasers, without sacrifice inbeam quality or other desirable laser characteristics.

What is claimed is:
 1. An elemental metal vapour laser comprising adischarge tube having a buffer gas therein and operating at a hightemperature such that the vapour pressure of said elemental metal is setby thermal evaporation of said metal and is sufficiently high to permitlaser light to be produced by said laser at said high temperature, saidbuffer gas including a laser output power enhancing substance in anamount sufficient to substantially increase the power output of saidlaser, wherein said laser output power enhancing substance is a speciescomprising one or more atoms selected from fluorine, chlorine, bromineand iodine.
 2. An elemental metal vapour laser comprising a dischargetube having a buffer gas therein, said buffer gas including a laseroutput power enhancing substance in an amount sufficient tosubstantially increase the power output of said laser, and meansoperatively associated with said discharge tube to add to said buffergas an additive capable of controlling the concentration of said laseroutput power enhancing substance in said buffer gas.
 3. An elementalmetal vapour laser according to claim 1 or claim 2, wherein said laseroutput power enhancing substance is selected from the group consistingof a mixture of hydrogen and fluorine, a mixture of hydrogen andchlorine, a mixture of hydrogen and bromine, a mixture of hydrogen andiodine, HF, HCl, HBr and HI.
 4. An elemental metal vapour laseraccording to claim 2, wherein said additive is selected from the groupconsisting of H₂, D₂, T₂, HD, HT, DT, H₂O and D₂O.
 5. An elemental metalvapour laser according to claim 4, wherein said additive is H₂ or H₂Oand said laser output power enhancing substance is selected from thegroup consisting of chlorine, bormine, HCl and HBr.
 6. An elementalmetal vapour laser according to claim 5, wherein said additive is H₂ andsaid laser output power enhancing substance is HCl.
 7. An elementalmetal vapour laser according to claim 1 or claim 2, wherein said laseroutput power enhancing substance is derived under operating conditionsof said laser from a metal halide included in said laser.
 8. Anelemental metal vapour laser according to claim 7, wherein said metalhalide is selected from the group consisting of CuF₂, CuCl₂, CuCl,CuBr₂, CuBr, CuI, AuCl₃, FeCl₃, HgBr₂, HgCl₂, HgF₂, NbBr₅, NbF₅, NbCl₅,OsF₅, TiCl₄, TiCl₃, TiBr₄, ZrCl₄, ZrBr₄, MoF₅, MoCl₅, NiCl₂, CoCl₂,WBr₅, WCl₅, WCl₆, AlCl₃, ReCl₅, ReCl₆, ReBr₄, PbBr₂, PbCl₂, TaCl₅, TaF₅,TaBr₅, TaI₅, SnBr₄, SnCl₂, SnCl₄, SnF₂, SnF₄, VCl₄, VCl₃, VCl₂, ZnBr₂,ZnBr₄ and mixtures of two or more thereof.
 9. An elemental metal vapourlaser according to claim 8, wherein said metal halide is a metalchloride.
 10. An elemental metal vapour laser according to claim 9,wherein said metal halide is TaCl₅ and/or ZrCl₄.
 11. An elemental metalvapour laser according to claim 5 which is a copper vapour laser,wherein said high temperature is from 1400-1700° C.
 12. An elementalmetal vapour laser according to claim 10 which is a copper vapour laser,wherein said high temperature is from 1400-1700° C.
 13. An elementalmetal vapour laser according to claim 7, wherein said metal halide isgenerated in said discharge tube by reaction of a metal and ahalogen-containing reagent.
 14. An elemental metal vapour laseraccording to claim 13, wherein said metal halide is generated in saiddischarge tube by reaction of copper and a halogen-containing reagent.15. An elemental metal vapour laser according to claim 13, wherein saidhalogen-containing reagent is a solid halogen-containing reagent.
 16. Anelemental metal vapour laser comprising a discharge tube and capable ofoperating at high temperature, said laser comprising a quantity of afirst metal capable of providing a sufficient metal vapour pressure atsaid high temperature to permit laser light to be produced by said laserat said high temperature wherein said high temperature is such that saidmetal vapour pressure is set by thermal evaporation of said metal,characterised in that said laser further comprises a quantity of asecond metal or a salt thereof, said second metal being different fromsaid first metal, said second metal or salt being capable of reactingwith a gaseous halogen-containing reagent to produce a halide of saidsecond metal, wherein a species derived from said halide of said secondmetal under an operating condition of said laser enhances the outputpower of said laser.
 17. An elemental metal vapour laser according toclaim 16, wherein said second metal is selected from the groupconsisting of tantalum, zirconium, palladium, nickel, niobium, platinum,copper, aluminum, titanium, molybdenum, tungsten, lead, rhenium and tin.18. An elemental metal vapour laser according to claim 16, wherein saidsecond metal is tantalum or zirconium.
 19. An elemental metal vapourlaser according to claim 16, wherein said gaseous halogen-containingreagent comprises a halogen or a hydrogen halide.
 20. An elemental metalvapour laser according to claim 19, wherein said gaseoushalogen-containing reagent is selected from the group consisting of HCl,HBr and mixtures thereof.
 21. An elemental metal vapour laser accordingto claim 16, wherein said second metal is present in intimate mixturewith a solid halogen-containing reagent capable of reacting with saidsecond metal under operating conditions of said laser to produce saidmetal halide.
 22. An elemental metal vapour laser according to claim 16,further comprising a quantity of a substance selected from the groupconsisting of a third metal and a metal halide, said third metal beingdifferent from said first and second metals.
 23. An elemental metalvapour laser according to claim 2, wherein said third metal is selectedfrom the group consisting of Au, Fe, Hg, Nb, Os, Ti, Zr, Mo, Ni, Co, W,Al, Re, Pb, Ta, Sn, V and Zn.
 24. An elemental metal vapour laseraccording to any one of claims 16-23, said laser containing a buffer gascomprising an additive which is hydrogen.
 25. An elemental metal vapourlaser according to any one of claims 16-23, said laser furthercomprising a quantity of a metal hydride capable of dissociating toproduce hydrogen atoms at an operating temperature of said laser.
 26. Anelemental metal vapour laser according to any one of claims 16-23,wherein said first metal is copper.
 27. An elemental copper vapourlaser, further comprising a quantity of tantalum metal or zirconiummetal, said laser including a buffer gas which is a mixture of HCl orHBr with an inert gas and hydrogen.
 28. A process for operating anelemental metal vapour laser comprising a discharge tube having a buffergas therein and operating at a temperature such that the vapour pressureof said elemental metal is set by thermal evaporation of said metal,wherein said metal vapour pressure is sufficiently high to permit laserlight to be produced by said laser at said temperature, comprisingpremixing a laser output power enhancing substance with said buffer gasand/or generating a laser output power enhancing substance in saiddischarge tube, said laser output power enhancing substance beingpresent in said discharge tube at an operating condition of said laserin an amount sufficient to substantially increase the power output ofsaid laser, wherein said laser output power enhancing substance is aspecies comprising one or more atoms selected from fluorine, chlorine,bromine and iodine.
 29. A process for operating an elemental metalvapour laser comprising a discharge tube having a buffer gas therein,said buffer gas including a laser output power enhancing substance in anamount sufficient to substantially increase the power output of saidlaser; comprising the step of adjusting the concentration of said laseroutput power enhancing substance by adding to said buffer gas anadditive capable of controlling the concentration of said laser outputpower enhancing substance in said buffer gas.
 30. A process according toclaim 28 or claim 29, wherein said laser output power enhancingsubstance is selected from the group consisting of a mixture of hydrogenand fluorine, a mixture of hydrogen and chlorine, a mixture of hydrogenand bromine, a mixture of hydrogen and iodine, HF, HCl, HBr and HI. 31.A process according to claim 29, wherein said additive is selected fromthe group consisting of H₂, D₂, T₂, HD, HT, DT, H₂O and D₂O.
 32. Aprocess according to claim 31, wherein said additive is H₂ or H₂O andsaid laser output power enhancing substance is selected from the groupconsisting of chlorine, boromine, HCl and HBr.
 33. A process accordingto claim 32, wherein said additive is H₂ and said laser output powerenhancing substance is HCl.
 34. A process according to claim 28 or claim31, wherein said laser output power enhancing substance is generated insaid discharge tube from a metal halide included in said laser.
 35. Aprocess according to claim 34, wherein said metal halide is selectedfrom the group consisting of CuF₂, CuCl₂, CuCl, CuBr₂, CuBr, CuI, AuCl₃,FeCl₃, HgBr₂, HgCl₂, HgF₂, NbBr₅, NbF₅, NbCl₅, OsF₅, TiCl₄, TiCl₃,TiBr₄, ZrCl₄, ZrBr₄, MoF₅, MoCl₅, NiCl₂, CoCl₂, WBr₅, WCl₅, WCl₆, AlCl₃,ReCl₅, ReCl₆, ReBr₄, PbBr₂, PbCl₂, TaCl₅, TaF₅, TaBr₅, TaI₅, SnBr₄,SnCl₂, SnCl₄, SnF₂, SnF₄, VCl₄, VCl₃, VCl₂, ZnBr₂, ZnBr₄ and mixtures oftwo or more thereof.
 36. A process according to claim 35, wherein saidmetal halide is a metal chloride.
 37. A process according to claim 36,wherein said metal halide is TaCl₅ and/or ZrCl₄.
 38. A process accordingto claim 29 wherein said laser is a copper vapour laser and saidtemperature is from 1400-1700° C.
 39. A process according to claim 37wherein said laser is a copper vapour laser and said temperature is from1400-1700° C.
 40. A process according to claim 34, wherein said metalhalide is generated in said discharge tube by reaction of a metal and ahalogen-containing reagent.
 41. A process according to claim 40, whereinsaid halogen-containing reagent is a solid halogen-containing reagent.42. A process according to claim 29, wherein the concentration of saidadditive is fixed and the concentration of said laser output powerenhancing substance is varied by varying a concentration of a precursorof said laser output power enhancing substance.
 43. A process accordingto claim 28 or 29, wherein said laser output power enhancing substanceis generated in said discharge tube, further comprising the steps of:pre-conditioning said laser by passing a gaseous halogen-containingreagent through said discharge tube at a temperature lower than saidhigh temperature for a time and under conditions sufficient for saidlaser output power enhancing substance or a precursor of said laseroutput power enhancing substance to be formed, or for said laser outputpower enhancing substance to be adsorbed or adsorbed on a surface ofsaid discharge tube, said reagent being capable of reacting with orbeing adsorbed or absorbed by said surface of said discharge tube;discontinuing passage of said gaseous halogen-containing reagent; andraising the temperature of said discharge tube to said high temperature.44. A process for operating an elemental metal vapour laser comprising adischarge tube having a buffer gas therein and operating at hightemperature such that the vapour pressure of said elemental metal is setby thermal evaporation of said metal, the process comprising: providingin said laser a quantity of a first metal and a quantity of a secondmetal or a salt thereof, said first metal being capable of providing asufficient metal vapour pressure at said high temperature to permitlaser light to be produced by said laser at said high temperature, saidsecond metal being different from said first metal, and said secondmetal or salt being capable of reacting with a gaseoushalogen-containing reagent to produce a halide of said second metalwherein a species derived from said halide of said second metal under anoperating condition of said laser enhances the output power of saidlaser; pre-conditioning said laser by contacting a gaseoushalogen-containing reagent with said second metal in said discharge tubeat a temperature lower than said high temperature for a time and underconditions sufficient for a halide of said second metal to be formed;raising the temperature of said discharge tube to said high temperature;passing a buffer gas through said discharge tube; and generating adischarge in said discharge tube and producing laser light from saidlaser.
 45. A process according to claim 44, wherein said second metal isselected from the group consisting of tantalum, zirconium, palladium,nickel, niobium, platinum, copper, aluminium, titanium, molybdenum,tungsten, lead, rhenium and tin.
 46. A process according to claim 45,wherein said second metal is tantalum or zirconium.
 47. A processaccording to claim 44, further comprising providing in said laser aquantity of a substance selected from the group consisting of a thirdmetal and a metal halide, said third metal being different from saidfirst and second metals.
 48. A process according to claim 44, whereinsaid gaseous halogen-containing reagent comprises a halogen or ahydrogen halide.
 49. A process according to claim 48, wherein saidgaseous halogen-containing reagent is selected from the group consistingof HCl, HBr and mixtures thereof.
 50. A process according to any one ofclaims 44-47, wherein said buffer gas comprises hydrogen.
 51. A processaccording to claim 50, wherein said buffer gas further comprises ahalogen or a hydrogen halide.
 52. A process according to any one ofclaims 44-47, wherein said first metal is copper.